As traffic congestion information, the following are provided to vehicle drivers: information about the positions of the forefront and tail end of congested sections, the overall lengths of those sections, a time it takes to go from the tail end to the forefront of each congested section, and the like. These pieces of information are provided through radio broadcasting, sign boards with lamps installed at major highways, automobile navigation systems equipped in vehicles, and the like.
Especially, in recent years, there have been introduced systems so constructed that vehicle sensors or the like for detecting running vehicles are installed on road sides and information obtained from the vehicle sensors is analyzed on the center side to identify a congested section or the like. (An example of such systems is vehicle information and communication system (VICS).) Drivers have come to be able to obtain detailed traffic congestion information, including the position of the forefront and the overall length of each congested section and the like, from these systems.                Patent Document 1: JP-A-10-96649 (corresponding to U.S. Pat. No. 6,006,161)        
When accurate traffic congestion information is provided, indeed, this information can be acquired and provided to users as-is. However, only relatively inaccurate traffic congestion information may be provided to users from external traffic information providing centers. In such a case, users' trust in information is relatively degraded. As an example, it is assumed that information indicating the degree of traffic congestion includes three types of “heavy traffic,” “slow traffic,” and “no congestion” is provided. In this case, the inaccurate information is, for example, such that: though information of “slow traffic” is acquired, the road is in “no congestion” state in actuality.